1. Field of the Invention
The present invention relates to a carrier for developing a latent electrostatic image suitable for use in electrophotography, electrostatic recording, electrostatic printing and other imaging processes and a method for producing such a carrier. The present invention further relates to a developer using such a carrier and an image forming method using the developer.
2. Description of the Related Art
The dry development process used in electrophotography is a process in which toner rubbed against a charged member is electrostatically attached to a latent electrostatic image-bearing member to form a visible image. There are two types of the dry development process: one is the single-component development system that primarily uses toner, and the other is the two-component development system that uses a mixture of toner and a carrier, such as glass beads, magnetic carrier or any of such carriers coated with a resin or other coatings.
A typical developer used in the two-component development system is composed of relatively large particles of a core material (i.e., carrier) that have smaller toner particles attached to their surface by the action of electrostatic force resulting from friction between the two components. As the developer comes in the proximity of a latent electrostatic image, the attraction acting on the toner that is caused by the electric field of the latent electrostatic image becomes stronger than the attraction between the toner and the core material. As a result, the toner is attracted to the latent electrostatic image, developing it into a visible image. Subsequently, the developer is repeatedly used by replacing the toner consumed during the development process.
Thus, the core material must be sufficiently charged by friction in order to retain a polarity that attracts toner during the extended use. However, collisions between the particles, mechanical stirring of the particles in the development apparatus and the heat generated in these processes can lead to a phenomenon known as “spent toner” in which the toner is fused to the surface of the core material. If spent toner occurs, the charge performance of the core material will decrease during use, resulting in smears in the resulting images and toner scattering. Consequently, the whole developer must be replaced.
One approach to prevent spent toner is by coating the surface of the core material with a fluorine resin, silicone resin or other resins that have a low surface energy. This significantly extends the life of the carrier. Examples of such carriers include a carrier coated with a room-temperature-setting silicone resin and a positively chargeable nitrogen resin (See, Japanese Patent Application Laid-Open (JP-A) No. 55-127569), a carrier coated with a coating material containing at least one modified silicone resin (See, JP-A No. 55-157751), a carrier having a coating layer containing a room-temperature-setting silicone resin and a styrene-acrylic resin (See, JP-A No. 56-140358), a carrier formed of core particles coated with two or more layers of silicone resin, the layers being not adhered to one another (See, JP-A No. 57-96355), a carrier formed of core particles surface-treated or coated with a polyvinyl acetal resin cross-linked with an isocyanate (See, JP-A No. 57-96356), a carrier coated with a silicone resin containing silicon carbide (See, JP-A No. 58-207054), a positively chargeable carrier coated with a material having a critical surface tension of 20 dyn/cm or less (See, JP-A No. 61-110161), and a developer composed of a carrier coated with a coating material containing a fluoroalkyl acrylate and a toner containing a chromium-containing azo dye (See, JP-A No. 62-273576).
Recently, smaller toner particles are being developed to further improve the quality of images. However, such smaller toner particles tend to fuse to carrier particles, leading to spent toner. In addition, the conventional spray coating technique is ineffective in uniformly wetting the carrier surface with a coating material, so that it is now difficult to manufacture carriers that have good adhesion between the coating layer and the core material and that have uniform coating thickness and uniform coating quality.
Furthermore, full-color toners tend to fuse to a carrier in larger amounts than black toners because of the low-softening point resins that are used in these toners to achieve sufficient color tones. This also leads to spent toner and can result in a decreased amount of charge that can be stored by the toner, as well as toner scattering and smears. The decrease in toner charge in a full-color electrophotographic system can result in varied image density in highlights, so that high image quality can no longer be maintained.
In general, when a carrier with high electrical resistance is used in a developer, the resulting image, especially when it is a large area photocopied image, would exhibit an “edge effect,” meaning that the image density is very low near the center of the image and high near the periphery of the image. While the edge effect serves to make an image sharp when the image is made up with characters and thin lines, medium tone images will have very poor reproducibility due to the same effect. For these reasons, there is a need to adequately control the electrical characteristics of carrier particles, in particular those used with full-color toners, in order to enable formation of high-quality medium tone images with sufficient color tones.
To improve the durability of carriers and facilitate formation of high-quality images, it has been proposed to control the anti-spent-toner property, coating strength and electrical characteristics of carrier by providing a coating layer composed of fine particles and a conductivity agent dispersed in a resin matrix of a low surface energy material (See, JP-A No. 09-319161, JP-A No. 09-269614, and JP-A No. 10-186731). However, each of these approaches involves applying a dispersion of the fine particles in an organic solvent at high temperature by spray coating. This causes the fine particles to aggregate, thus making it difficult to achieve uniform charge.
It has also been proposed to control the electrical resistance of the carrier surface by depositing on the surface of core material a coating layer composed of a conductive polymer to serve as a conductivity agent dispersed in a resin (Japanese Patent (JP-B) No. 2626754). According to this approach, however, the undesirable compatibility between the resins makes it difficult to disperse the conductive polymer in the coating layer uniformly enough to achieve stable resistance.
In addition, organic solvents used in conventional spray coating to deposit the coating layer on the surface of core material generate volatile organic chemicals (VOCs), for which regulations have been established. Conventional spray coating has additional problems associated with it, such as liquid wastes and energy required to dry the coating.
One approach to these problems is to produce carrier particles in an organic solvent-free, dry powder process. One such process uses a supercritical fluid (See, U.S. Pat. No. 5,514,512). However, this process, in which a coating resin polymerized in a supercritical fluid is melted by heating for coating, does not provide a solution to the problem of spent toner since the coating resin is an acrylic resin.
JP-A No. 2006-106208 also concerns a production method of carriers using a supercritical fluid in which the coating material is a silicone resin. In this method, the poor solubility of the silicone resin in the supercritical fluid makes it necessary to plasticize and micro-disperse the silicone resin. The dispersed resin is sprayed with a core material to coat the core material with the resin. The resulting coating layer has non-uniform thickness and is not thick enough to achieve the desired durability. Moreover, a conductivity agent is dispersed in the resin matrix by kneading, so that the conductivity agent is not thoroughly micro-dispersed. Thus, this method fails to provide a solution to the above-described problem of non-uniform composition caused by aggregation.
Therefore, no satisfactory technology has been developed to date for the production of carriers that have a coating layer with uniform thickness and achieve good adhesion between the core material and the coating layer, nor has any relevant technology been developed. From the viewpoint of environmental load and saving resources, there are still many problems associated with conventional carriers and production methods thereof that need to be addressed.